The present invention pertains to relay control circuits. More particularly, it pertains to a means for providing control signals for a latching relay and for resetting said relays when power is interrupted.
Power relays of substantial current-handling capability, approximately two amp inductive loads, are required for HVAC (heating, ventilation and air conditioning) control. The relays available of the continuously powered kind require a substantial holding current to operate. It is desirable to be able to operate a control system on a small power supply to eliminate the need for active heat removal and, if possible, permit the use of a battery backup power supply of modest size. Since a control system may need to control multiple relays, the level of power for conventional relays cannot be made available from a small power supply system.
Latching relays have the benefit of only requiring power when they need to change state. The difficulty in using these relays is they need to be given a deliberate timed pulse to open or close. Further, their state is unknown when a power failure occurs. Thus, it is desired to find a simple means of providing a means to force the relays to the off position upon a power failure or at initial power-up.